High capacity aircraft are equipped with hydraulic circuits to distribute energy necessary for systems requiring high power levels, such as devices for manipulating the landing gear, braking system or flight controls.
For safety reasons, three rigorously independent hydraulic circuits are provided for such aircraft and in certain cases, four. This is described in a reliability course given by Mr. Jacques de CORLIEU (pp 91 to 105; 1971) to the students of the Ecole Superieur de l'Aeronautique et de l'Espace. The monitoring and control functions with respect to such hydraulic circuits are not grouped in a single computer. Thus, on one aircraft type, these functions are provided by electric circuits integrating relay logics. The use of relays leads to a certain complexity of the installation as soon as it is necessary to have logics including numerous parameters. In another aircraft type, four computers are used just for monitoring the hydraulic circuits (one per circuit) without performing a control function. The control functions are carried out by the crew from a control panel located in the flight deck or cockpit. Using relays in an aircraft makes it impossible to provide complex logic automatic operations and an analysis and cohesion between different information obtained from the monitored systems. This is due to the installation and weight problems due to use of the relays. Accordingly, this analysis is carried out by the crew, which must make a decision to carry out necessary actions as a function of each situation. Although the procedures are well known to pilots, in certain situations, the reaction time is too long or an incorrect choice is made.
Moreover, the maintenance personnel for the hydraulic circuits must be well informed on the precautions which must be observed before and during the pressurization of a circuit, so as to avoid potential accidents. The search for possible malfunctions is time consuming and requires, apart from a good knowledge of the system, a good description of the detected fault and an expert analysis. If the maintenance personnel have to deal with several different aircrafts from different manufacturers, more documents must be consulted. Assistance in maintenance by identification and storage of a fault is provided by the "Built in Test Equipment" or "Bite", which mainly consists of recording the malfunction in a memory, whose content is then analyzed on the ground by maintenance specialists. The malfunctions detected during operation are not integrated into the relay logics of hydraulic circuits of prior art aircraft.